The present invention relates generally to optical instruments and, more particularly, to a method and device for defocus and astigmatism compensation in wavefront aberration measurement systems. The present invention is particularly useful, but not exclusively so, for defocus and astigmatism compensation in ophthalmic applications.
The human eye is an optical system employing several lens elements to focus light rays representing images onto the retina within the eye. The sharpness of the images produced on the retina is a factor in determining the visual acuity of the eye. Imperfections within the lens and other components and material within the eye, however, may cause the light rays to deviate from the desired path. These deviations, referred to as aberrations, result in blurred images and decreased visual acuity. Hence, methods and apparatuses for measuring aberrations are desirable to aid in the correction of such problems.
One method of detecting aberrations introduced by the eye involves determining the aberrations of light rays exiting from the eye. A beam of light directed into the eye as a point on the retina is reflected or scattered back out of the eye as a wavefront, with the wavefront containing aberrations introduced by the eye. By determining the propagation direction of discrete portions (i.e., samples) of the wavefront, the aberrations introduced by the eye can be determined and corrected.
A general illustration of the generation of a wavefront is shown in FIG. 1. FIG. 1 is a schematic view of a wavefront 10 generated by reflecting a laser beam 12 off of the retina 14 of an eye 16. The laser beam 12 focuses to a small spot 18 on the retina 14. The retina 14, acting as a diffuse reflector, reflects the laser beam 12, resulting in the point source wavefront 10. Ideally, the wavefront 10 would be represented by a planar wavefront 20. However, aberrations introduced by the eye 16 as the wavefront 10 passes out of the eye 16 result in an imperfect wavefront, as illustrated by the aberrated wavefront 20A. The wavefront 10 represents aberrations which lead to defocus, astigmatism, spherical aberrations, coma, and other irregularities. Measuring and correcting these aberrations allow the eye 16 to approach its full potential, i.e., the limits of visual resolution.
FIG. 2 is an illustration of a prior art apparatus for measuring the wavefront 10 as illustrated in FIG. 1. By measuring the aberrations, corrective lenses can be produced and/or corrective procedures performed to improve vision. In FIG. 2, a laser 22 generates the laser beam 12 which is routed to the eye 16 by a beam splitter 24. The laser beam 12 forms a spot 18 on the retina 14 of the eye 16. The retina 14 reflects the light from the spot 18 to create a point source wavefront 10 which becomes aberrated as it passes through the lens and other components and materials within the eye 16. The wavefront 10 then passes through a first lens 11 and a second lens 13 to focus the wavefront 10 so that the wavefront 10 is collimated. The wavefront 10 then passes through the beam splitter 24 toward a wavefront sensor 26. Information detected by the wavefront sensor 26 is then processed by a processor 27 to determine the aberrations of the wavefront 10.
FIG. 3 illustrates the focusing of the wavefront 10 to produce a flat wavefront for projection onto the wavefront sensor 26. If the wavefront 10 contains diverging light, the light rays which make up the wavefront 10 would continue to diverge until they were no longer contained within the system, thereby losing valuable wavefront 10 information. This is especially problematic for an eye 16 having a large degree of defocus. In FIG. 3 the curved wavefront 10A containing diverging light rays passes through the first lens 11 where it converges to a crossover point 15, and then through the second lens 13. When the crossover point 15 occurs at one focal length before the second lens 13, the resultant wavefront 10B will be collimated (i.e., flat). For different degrees of defocus, the lenses 11 and 13 can be moved relative to one another in order for the focal point of lens 13 to match the cross-over point 15. Unfortunately, for an eye 16 having a great deal of defocus, the lenses 11 and 13 may need to be moved a relatively large distance from one another, which may be problematic if space is limited. In addition, the defocus mechanism of FIG. 3 does not correct other eye aberrations such as astigmatism in which light along one axis converges/diverges more rapidly than light along another axis. Since the lenses 11 and 13 converge or diverge light along every axis equally, this arrangement does not compensate for astigmatism.
Typical wavefront sensors 26 include either an aberroscope 28 (FIG. 4) or a Hartman-Shack lenslet array 30 (FIG. 5), with an imaging device 32. The aberroscope 28 and the Hartman-Shack lenslet array 30 each produce an array of spots when a wavefront passes through them. The imaging device 32 contains an imaging plane 34 for capturing the spots generated by the aberroscope 28 or the Hartman-Shack Sensor 30. Generally, the imaging device 32 is a charge coupled device (CCD) camera.
The wavefront sensor 26 samples the wavefront 10 by passing the wavefront 10 through the aberroscope 28 or the Hartman-Shack sensor 30, resulting in an array of spots on the imaging plane 34. Each spot on the imaging plane 34 represents a portion of the wavefront 10, with smaller portions enabling the aberrations to be determined with greater accuracy. By comparing the array of spots produced on the imaging plane 34 by the wavefront 10 with a reference array of spots corresponding to the wavefront of an ideal eye, the aberrations introduced by the eye 16 can be computed.
An example of a Hartman-Shack system is described in U.S. Pat. No. 6,095,651 to Williams et al., entitled Method and Apparatus for Improving Vision and the Resolution of Retinal Images, filed on Jul. 2, 1999, is incorporated herein by reference.
The resolution of the aberrations in such prior art devices, however, is limited by the sub-aperture spacing 36 and the sub-aperture size 38 in an aberroscope sensor (see FIG. 4), and by the lenslet sub-aperture size 40 and focal length in a Hartman-Shack sensor (see FIG. 5). In addition, large aberrations due to excessive defocus or astigmatism may result in foldover. Foldover occurs in an aberroscope sensor, for example, when two or more spots 42A, 42B, and 42C on the imaging plane 34 overlap, thereby leading to confusion between adjacent sub-aperture spots. Similarly, foldover occurs in Hartman-Shack sensors when two or more spots 44A, 44B, 44C, and 44D on the imaging plane 34 overlap. Typical systems are designed to accommodate a certain amount of defocus and astigmatism, however, these systems are unable to handle defocus and astigmatism of individuals with large astigmatism and/or large defocus.
Foldover may result from a sub-aperture spacing 36, sub-aperture size 38, or lenslet size 40 which is too small, a high degree of aberration (e.g., large defocus and/or astigmatism); or a combination of these conditions. Hence, the sub-aperture spacing 36 and sub-aperture size 38 in the aberroscope sensor (FIG. 4), and the lenslet sub-aperture spacing 40 and focal length in the Hartman-Shack sensor (FIG. 5) must be selected to achieve good spatial resolution while enabling the measurement of large aberrations. Accordingly, the ability to measure a high degree of aberration comes at the expense of spatial resolution and/or dynamic range and vice versa.
The constraints imposed by the aberroscope and Hartman-Shack approaches limit the effectiveness of these systems for measuring wavefronts having a wide range of aberrations, such as those exhibiting a large degree of defocus and astigmatism. These limitations prevent existing optical systems from achieving their full potential. Accordingly, ophthalmic devices and methods which can measure a wide range of aberrations having of defocus and/or astigmatism with a high degree of accuracy would be useful.
The present invention provides for a method and apparatus of compensating for defocus and astigmatism in a wavefront for use in an ophthalmic system for measuring eye aberrations. By compensating for at least a portion of defocus and astigmatism, the method and apparatus of the present invention are capable of measuring a wide range of aberrations in a wavefront with a high degree of accuracy.
In an ophthalmic system for measuring eye aberrations having first and second optical lenses separated by a physical distance for focusing a wavefront, the present invention includes a method of adjusting the optical distance between the two lenses without changing the physical distance between the two lenses. The method of the present invention includes passing a wavefront through a first optical lens in a first optical path, reflecting the wavefront from the first optical path to a second optical path, reflecting the wavefront to a third optical path, and passing the wavefront through a second optical lens. In addition, the method may include reflecting the wavefront to a fourth optical path after being reflected to the third optical path and before being passed through the second optical lens. The reflections allow the optical distance between the first and second optical lenses, and therefore the defocus compensation, to be changed without altering the physical distance between the lenses. Also, the reflections allow incremental changes in certain components to result in larger incremental changes in the optical distance between the lenses, thereby allowing a larger range of defocus compensation to be performed in a smaller physical area.
Another method of the present invention includes passing a wavefront through a cylindrical lens assembly to remove astigmatism from the wavefront. The method includes passing the wavefront through a first cylindrical lens and a second cylindrical lens, orienting the first cylindrical lens and the second cylindrical lens such that an astigmatism compensation position of the cylindrical lens assembly is in-line with a bisector position of the wavefront, and orienting the first and second cylindrical lenses relative to one another to adjust the astigmatism compensation power of the cylindrical lenses to compensate for astigmatism in the wavefront.
An apparatus of the present invention for changing the optical distance traveled by a wavefront between a pair of lenses without changing the physical distance between the lenses includes a first reflector positioned to reflect the wavefront received from a first lens along a first optical path to a second optical path, a second reflector positioned to reflect the wavefront from the second optical path to a third optical path, and a third reflector positioned to reflect the wavefront from the third optical path to a fourth optical path which passes through a second optical lens.
An apparatus of the present invention for compensating for astigmatism includes a first cylindrical lens for introducing a first cylindrical refraction to a wavefront, a second cylindrical lens for introducing a second cylindrical refraction to the wavefront, and a support for rotatably mounting the first and second cylindrical lenses, the first and second cylindrical lenses being rotatable relative to the wavefront and relative to one another, whereby an astigmatism within the wavefront is compensated by adjusting the orientation of the first cylindrical lens and the second cylindrical lens relative to the wavefront and to one another.